This invention relates to a membrane which shows only a small reduction of water permeability during the time of hot water treatment or drying treatment and a method for producing such a membrane.
A membrane which is constructed with a porous substance at least one part of which substance forms porous network region having pore sizes in the range of 500A - 5.mu. and containing numbers of voids having a diameter greater than 10.mu. in said porous region, has heretofore been known as a semipermeable membrane which shows higher water permeability when it is used as it is in the wet state and at a lower temperature (see U.S. Pat. No. 3,871,950 one of which inventors is same as one of those of the present application).
The structure of such a membrane can be observed by way of electronmicroscopic photograph of the cross-section of membrane and the porous substance is made of a polymer. The void herein referred to means a part of the porous substance at which the porous substance is lacking in an area corresponding to a circle having a diameter greater than 10.mu., and the porous region of network structure means a region of the porous substance made of a polymer existing while surrounding numbers of above-mentioned voids and forming network structure by the mutual connection of adjacent pores having diameters in the range of 500A - 5.mu..
In addition to collodion membranes, gel-cellophane membranes, etc., membranes of many kinds of high molecular weight polymers such as collagen membranes, dextrane membranes acrylonitrile polymer membranes have been brought into application recently and their use is now under development not only in the laboratory scale but also in a wide variety of application fields such as foodstuff industries, medicals industries, electronics industries, etc. as well as in the public nuisance prevention art.
With regard to the shapes of such membranes, there are many kinds such as flat membranes obtained by casting a polymer solution upon a flat base plate followed by removing the solvent (for the polymer), tubular membranes obtained by coating a cylindrical supporting material with a polymer solution, also hollow fiber membranes obtained by spinning a polymer solution through a spinning nozzle having a circular orifice, etc.
However, any of these membranes has restriction in the actual use because the water permeability is extremely reduced and/or the shape of membrane cannot be maintained when they are brought into contact with hot water or they are dried.
Accordingly, it is an object of the present invention to provide membranes which are very small in the reduction of water permeability even by the hot water treatment or drying treatment without injuring the characteristic feature of the past membrane which has a higher water permeability due to possession of a large number of voids as well as a porous region of network structure as compared with the membranes having no such a region.
It is another preferable object of the present invention to provide microporous ultrafilatration membranes.
After repetition of a comprehensive experimentation and deliberation, we have found semipermeable membranes having superior resistance to hot water and to drying which show a ratio of the water permeability in one time treatment with hot water at 80.degree. C or drying at 20.degree. C to the water permeability before the treatment, of 0.3 or higher. Such superior value has never been experienced in case of conventional membranes. These are the membranes which use a fabric as reinforcing material in case of flat type, and a braid having a central hollow portion as reinforcing material in case of hollow fiber type. Further a rounded fabric can be also used as reinforcing material to form a membrane of a tubular type. In these membranes, these reinforcing materials are placed in the wall portion of membranes; at least one part of the porous substance constituting the membranes forms a porous region of network structure having pore sizes in the range of 500A - 5.mu., and a large number of voids having diameters greater than 10.mu. are formed in said porous region. Further, objective membranes can not be obtained from those having at least a portion of these reinforcing materials outside the wall portion of the membrane or those in which the porous substance constituting the membrane is different from the above-mentioned.
In order to give understanding more fully of the nature of the membranes of the present invention and the process for producing the same, a process for producing a semipermeable composite membrane of hollow fiber type containing a braid will be illustrated as one embodiment of the present invention.